1. Field of the Invention
The present invention relates to a CS (Communications Satellite) broadcast receiving system for receiving CS television broadcasting and CS sound broadcasting.
2. Description of the Related Art
In CS television broadcasting in practice today, a picture signal is generated by FM modulation and a subcarrier at 5.73 MHz is set up to generate a sound signal on the QPSK modulation system. The sound signal is superimposed on the picture signal and the thus obtained signal is broadcast as the CS television signal.
On the other hand, in CS sound broadcasting, digital data at 24 Mbps is MSK-modulated as is and the thus obtained signal is broadcast as the CS sound signal.
There is not existing at present a television-and-sound receiver capable of receiving both the CS television broadcasting and the CS sound broadcasting. We find there are the following problems when we actually produce such a receiver.
In CS television broadcasting, it is not a problem even if the frequency of an AFC circuit (Automatic Frequency Control circuit) drifts ".+-.500 KHz" or so with respect to the carrier frequency, and hence the frequency adjustment step in the AFC circuit can be set to "100 KHz" or so. In the CS sound broadcasting, however, the carrier accuracy of the MSK demodulator must be kept within ".+-.100 KHz" or so. Accordingly, the frequency adjustment step in the AFC circuit must be set to a frequency between "10 KHz" and "several tens of KHz".
Therefore, when an AFC circuit is so arranged as to be commonly used for the receiving circuit of the CS television broadcasting and the receiving circuit of the CS sound broadcasting, the frequency adjustment step in the AFC circuit must be set to a frequency between "10 KHz" and "several tens of KHz" so that the frequency accuracy of the AFC circuit required by the CS television broadcasting and the frequency accuracy of the AFC circuit required by the CS sound broadcasting are both satisfied.
However, while there is no problem when receiving the CS sound broadcasting with the described method, there arises a problem when receiving the CS television broadcasting because a period of time more than necessary is taken, due to the over-specification, before the receiver tunes in to a desired station, and it causes users to feel dissatisfied with the receiver.